OFDMA Communication System and Communication Method

ABSTRACT

There is provided an OFDMA communication system capable of suppressing the communication resource reduction and reducing the processing load on a base station. The system includes a downlink frame generation unit ( 14 ) that generates a downlink frame for a downlink period for performing communication to at least one terminal ( 20 ) of a plurality of terminals from the base station ( 10 ), an uplink frame generation unit ( 24 ) that generates an uplink frame for an uplink period for performing communication to the base station ( 10 ) from at least one terminal ( 20 ) of the plurality of terminals, and a channel allocation unit that allocates, for one terminal of the plurality of terminals, one or more subchannels available in the one terminal, wherein the channel allocation unit allocates, among the subchannels, subchannels having a same time slot and a same frequency band in the downlink frame and in the uplink frame for a single terminal of the plurality of terminals.

TECHNICAL FIELD

The present invention relates to an OFDMA communication system andcommunication method.

BACKGROUND ART

As a wireless access scheme of a digital portable telephone system, aPHS system, and the like, a TDMA (Time Division Multiple Access)/TDD(Time Division

Duplex) scheme in which TDMA and TDD are combined has been adopted.Recently, an OFDMA (Orthogonal Frequency Division Multiplexing Access)scheme using OFDMA based on a technique of OFDM (Orthogonal FrequencyDivision Multiplexing) has been proposed.

The OFDM is a scheme of dividing a carrier for data modulation into aplurality of “subcarriers” (subdivided carriers) orthogonal to eachother and distributing and transmitting a data signal in eachsubcarrier.

Hereinafter, the overview of the OFDM scheme will be described.

FIG. 8 is a block diagram showing a configuration of an OFDM modulationdevice to be used at a transmitting side. Transmission data is input tothe OFDM modulation device. The transmission data is supplied to aserial/parallel conversion unit 201 and converted into data including aplurality of low-speed transmission symbols. That is, a plurality oflow-speed digital signals are generated by dividing transmissioninformation. Parallel data is supplied to an inverse fast Fouriertransform (IFFT) unit 202.

The parallel data is allocated to each subcarrier configuring OFDM andmapped in a frequency domain. Here, each subcarrier is modulated byBPSK, QPSK, 16 QAM, 64 QAM, and the like. The mapping data istransformed from frequency-domain transmission data to time-domaintransmission data by performing an IFFT operation. Thereby, multicarriermodulation signals into which a plurality of subcarriers orthogonal toeach other are modulated independently are generated. An output of theIFFT unit 202 is supplied to a guard interval adding unit 203.

As shown in FIG. 9, the guard interval adding unit 203 sets a rear partof an effective symbol of transmission data as a guard interval and addsits copy to a front part of an effective symbol period for everytransmission symbol. A base-band signal obtained by the guard intervaladding unit is supplied to an orthogonal modulation unit 204.

The orthogonal modulation unit 204 orthogonally modulates a base-bandOFDM signal supplied from the guard interval adding unit 203 using acarrier signal supplied from a local oscillator 105 of the OFDMmodulation device, and performs frequency conversion into anintermediate frequency (IF) signal or a radio frequency (RF) signal.That is, after frequency-converting the base-band signal into a desiredtransmission frequency band, the orthogonal modulation unit outputs itto a transmission path.

FIG. 10 is a block diagram showing a configuration of an OFDMdemodulation device to be used at a receiving side. An OFDM signalgenerated by the OFDM modulation device of FIG. 8 is input to the OFDMdemodulation device through a predetermined transmission path.

An OFDM reception signal input to the OFDM demodulation device issupplied to an orthogonal demodulation unit 211. The orthogonaldemodulation unit 211 orthogonally demodulates the OFDM reception signalusing a carrier signal supplied from a local oscillator 212 of the OFDMdemodulation device, performs frequency conversion from an RF signal oran IF signal to a base-band signal, and obtains a base-band OFDM signal.The OFDM signal is supplied to a guard interval removing unit 213.

The guard interval removing unit 213 removes a signal added by the guardinterval adding unit 203 of the OFDM modulation device according to atiming signal supplied from a symbol timing synchronizing unit (notshown). A signal obtained by the guard interval removing unit 203 issupplied to a fast Fourier transform (FFT) unit 214.

The FFT unit 214 performs transformation to frequency-domain receptiondata by performing an FFT operation on input time-domain reception data.De-mapping is performed in the frequency domain and parallel data isgenerated for each subcarrier. Here, the demodulation to the modulationof BPSK, QPSK, 16 QAM, 64 QAM, etc. performed for each subcarrier isperformed. Parallel data obtained by the FFT unit 214 is supplied to aparallel/serial conversion unit 215 and output as reception data.

The above-described OFDM is a scheme for dividing a carrier into aplurality of subcarriers. The OFDMA is a scheme for collecting andgrouping a plurality of subcarriers among the subcarriers in theabove-described OFDM and performing multiplex communication byallocating one or more groups to each user. Each group is called asubchannel. That is, each user performs communication using one or moresubchannels allocated. According to a communication data amount, apropagation environment, and the like, subchannels are adaptivelyincreased/decreased and allocated.

Next, an example of channel configuration of a communication systemadopting the OFDMA scheme will be described.

Patent Document 1 describes a communication method based on asymmetricchannels with different bandwidths. In the communication method,downstream line (downlink) communication is performed by a broadbandchannel, and upstream line (uplink) communication is performed by anarrowband channel

FIG. 11 is a configuration of transmission control between a terminaldevice and a base station in Patent Document 1. An OFDMA scheme isapplied as an access scheme and different time slots within one frameare used by time division in the upstream line and the downstream line.

A predetermined number of slots T1, T2, . . . , Tn (where n is anarbitrary integer) of the first half of one frame are slots of an uplinkperiod Tu as slots to be used for upstream line transmission from theterminal device to the base station. A predetermined number of slots R1,R2, . . . , Rn (where n is an arbitrary integer) of the second half ofone frame are slots of a downlink period Td as slots to be used fordownstream line transmission from the base station to the terminaldevice. As described above, a frame in which the uplink period and thedownlink period are different from each other (times of the upstream anddownstream are different from each other and slots configuring theupstream and downstream are different from each other) is referred to asan up-down asymmetric frame.

FIG. 12 is an example of channel configuration in which data having theabove-described frame configuration is transmitted wirelessly.

In this example, guard band parts B1 and B2 narrower than bandwidths ofbroadband channels CH1 to CH4 exist at an upper side and a lower side ofan available frequency band B0. B1 and B2 are arranged with narrowbandchannels CH5 and CH6 which are narrower than the broadband channels CH1to CH4, respectively.

The narrowband channels CH5 and CH6 arranged in the guard band parts areused as dedicated communication channels for low-speed access in theupstream line (uplink), and only the uplink period Tu of the first halfof the frame configuration shown in FIG. 11 is used for wirelesstransmission.

Patent Document 2 describes a communication method in whichcommunication between a base station and a mobile station is performedby allocating a time slot to be used in each communication counterparton the basis of a situation of a transmission waiting cell for each ofthe downstream line (downlink) and the upstream line (uplink), and acommunication device adopting art OFDMA/TDD scheme for allocating a userchannel according to a transmission/reception amount and QoS of eachasymmetric channel.

FIG. 13 is a schematic diagram showing a configuration of acommunication system of Patent Document 2. Communication adopting theOFDMA scheme is performed between a base station (BTS) and a mobilestation (MS).

FIG. 14 is a schematic diagram showing the format of a frame to be usedin a wireless communication device of Patent Document 2. As shown, aunit frame (1 frame) includes an access channel (Ach), a control channel(Cch) of an upstream direction, a control channel (Cch) of a downstreamdirection, a user channel (Uch) of the downstream direction, and a userchannel (Uch) of the upstream direction.

The number of time slots including each of the user channel of thedownstream direction and the user channel of the upstream direction isnot fixed, and a boundary position is determined on the basis of a userchannel allocation result.

Patent Document 1: JP-A-2000-115834

Patent Document 2: JP-A-2000-236343

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

In a communication system adopting the conventional OFDMA scheme asdescribed above, information indicating which subchannel is allocated towhich terminal for communication with a base station is referred to asMAP information and pre-notified from the base station to each terminal.In the conventional OFDMA scheme, channel configuration of thedownstream line (downlink) and the upstream line (uplink) are asymmetricframe configuration. Thus, in the above-described communication system,MAP information for each of a plurality of terminals needs to betransmitted MAP information for a downlink frame and MAP information foran uplink frame separately.

However, since MAP information of each of the downlink frame and theuplink frame has to be transmitted, an information amount of the MAPinformation is large and a communication resource of the informationamount is reduced.

Additionally, a problem exists in that the processing load on the basestation for determining the MAP information increases.

The present invention has been made to solve the above-described problemand provides an OFDMA communication system and communication method thatcan suppress the communication resource reduction and reduce theprocessing load on a base station.

Means for Solving the Problem

To solve the above-described problem, a communication system accordingto the present invention is an OFDMA communication system for performingcommunication using one or more subchannels between a base station and aplurality of terminals, the communication system comprising: a downlinkframe generation unit which generates a downlink frame for a downlinkperiod for performing communication from the base station to at leastone terminal of the plurality of terminals; an uplink frame generationunit which generates an uplink frame for an uplink period for performingcommunication from at least one terminal of the plurality of terminalsto the base station; a channel allocation unit which allocates, for oneterminal of the plurality of terminals, one or more subchannelsavailable in the one terminal, wherein the channel allocation unitallocates, among the subchannels, subchannels having a same time slotand a same frequency band in the downlink frame and in the uplink framefor a single terminal of the plurality of terminals (claim 1).

Further, a number of subchannels configuring the downlink frame is sameas a number of subchannels configuring the uplink frame (claim 2).

Further, the downlink frame and the uplink frame are arrangedcontinuously (claim 3).

Further, each of the downlink frame and the uplink frame includes acontrol subchannel to be used as a control channel of the base stationand a traffic subchannel for transmitting data, and wherein the trafficsubchannel includes: a first subchannel which is allocated for eachterminal and includes information indicating an available or unavailablesubchannel for each terminal; and a second subchannel including data tobe used substantially (claim 4).

Further, the information indicating an available or unavailablesubchannel for each of the plurality of terminals is included in thefirst subchannel and notified from the base station to the correspondingterminal in the downlink period (claim 5).

Further, among the available subchannels in the information, asubchannel to be used and a subchannel to be unused in each terminal areincluded in the first subchannel while being differentiated from eachother, and notified from the corresponding terminal to the base stationin the uplink period (claim 6).

Further, a communication method according to the present invention is anOFDMA communication method for performing communication using one ormore subchannels between a base station and a plurality of terminals,the communication method comprising: generating a downlink frame for adownlink period for performing communication from the base station to atleast one terminal of the plurality of terminals, and an uplink framefor an uplink period for performing communication from at least oneterminal of the plurality of terminals to the base station; andallocating, for one terminal of the plurality of terminals, one or moresubchannels available in the one terminal, wherein communication isperformed by allocating, among the subchannels, subchannels having asame time slot and a same frequency band in the downlink frame and inthe uplink frame for a single terminal of the plurality of terminals(claim 7).

Further, a number of subchannels configuring the downlink frame is sameas a number of subchannels configuring the uplink frame (claim 8).

Further, the communication method further comprises notifyinginformation indicating an available or unavailable subchannel for eachof the plurality of terminals to the corresponding terminal in thedownlink period (claim 9).

Further, the communication method further comprises, after notifying theinformation to each terminal in the downlink period, notifying, amongthe available subchannels, a sub channel to be used and a subchannel tobe unused in each terminal while being differentiated from each other,from the corresponding terminal to the base station in the uplink period(claim 10).

Advantage of the Invention

In an OFDMA communication system and communication method according tothe present invention, the communication resource reduction can besuppressed. The processing load on a base station can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing transmission functions of a basestation and a terminal in a communication system according to anembodiment of the present invention.

FIG. 2 is an illustrative diagram showing an OFDMA frame configurationto be used in a communication method according to an embodiment of thepresent invention.

FIG. 3 is an illustrative diagram showing one example of a MAPconfiguration in the frame of FIG. 2.

FIG. 4 is an illustrative diagram showing the format of a subchannel.FIG. 5 is an illustrative diagram showing the format of a downlinkphysical layer (PHY).

FIG. 6 is an illustrative diagram showing the format of an uplinkphysical layer

(PHY).

FIG. 7 is an illustrative diagram showing a frame corresponding totransmitted MAP information. FIG. 8 is a block diagram showing aconfiguration of an OFDM modulation device to be used at a transmittingside.

FIG. 9 is an illustrative diagram showing a guard interval.

FIG. 10 is a block diagram showing a configuration of an OFDM modulationdevice to be used at a receiving side.

FIG. 11 is a configuration diagram of transmission control between aterminal device and a base station of Patent Document 1.

FIG. 12 is an example of a channel configuration in which data of theframe configuration of FIG. 11 is transmitted wirelessly.

FIG. 13 is a schematic diagram showing a configuration of acommunication system of Patent Document 2.

FIG. 14 is a schematic diagram showing the format of a frame to be usedin a wireless communication device of Patent Document 2.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

10: BASE STATION

11, 21: QoS CONTROL UNIT

12, 22: SCHEDULER

13, 23: BAND ALLOCATION UNIT

14: DOWNLINK FRAME GENERATION UNIT

15, 25: MODULATION UNIT

16, 26: TRANSMISSION UNIT

17, 27: COMMUNICATION MANAGEMENT UNIT

20: TERMINAL

24: UPLINK FRAME GENERATION UNIT

S1 to S4: TIME SLOT

C₁ to C₄: CONTROL SUBCHANNEL

T₁ to T₁₀₈: TRAFFIC SUBCHANNEL

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of a communication system according to thepresent invention will be described in detail with reference to thedrawings.

This communication system is an OFDMA communication system forperforming communication by a frame including a plurality of subchannelsfor each frequency band between a base station (CS: cell station) and aplurality of terminals (PS: personal station). FIG. 1 is a block diagramshowing transmission functions of a base station and a terminal in acommunication system according to an embodiment of the presentinvention.

As shown in FIG. 1, a base station includes, as transmission functions,a QoS control unit 11 for dividing a QoS class for data transmitted froman upper layer according to a communication priority, a scheduler 12 forscheduling communication according to the priority by which the class isdivided, a band allocation unit 13 for allocating a subchannel describedlater to each slot, a downlink frame generation unit 14 for generating adownlink frame for a downlink period for performing communication to aterminal 20, a modulation unit 15 for modulating a downlink framesignal, a transmission unit 16 for transmitting a radio signal to theterminal, and a communication management unit 17 for managingcommunication by controlling the band allocation unit 13 and themodulation unit 15. The downlink frame generation unit 14 generates thedownlink frame by four continuous physical frames transmitted from theupper layer through the QoS control unit 11 and the scheduler 12 andallocated to each subchannel through the band allocation unit 13.

A terminal 20 includes, as transmission functions, includes a QoScontrol unit 21 for dividing a QoS class for data transmitted from anupper layer according to a communication priority, a scheduler 22 forscheduling communication according to the priority by which the class isdivided, a band allocation unit 23 for allocating a subchannel describedlater to each slot, an uplink frame generation unit 24 for generating anuplink frame for an uplink period for performing communication to thebase station 10, a, modulation unit 25 for modulating an uplink framesignal, a transmission unit 26 for transmitting a radio signal to thebase station, and a communication management unit 27 for managingcommunication by controlling the band allocation unit 23 and themodulation unit 25. The uplink frame generation unit 24 generates theuplink frame by four continuous physical frames transmitted from theupper layer through the QoS control unit 21 and the scheduler 22 andallocated to each subchannel through the band allocation unit 23.

FIG. 2 is an illustrative diagram showing an OFDMA frame configurationto be used in a communication method according to an embodiment of thepresent invention.

The frame is arranged so that a time slot of the downlink period forperforming communication from the base station to the terminal and atime slot of the uplink period for performing communication from theterminal to the base station are adjacent to each other.

In a frame configuration indicating the allocation of a plurality ofsubchannels in the above-described frame, the downlink frame as a frameof the period of the downlink (link from the base station to theterminal) and the uplink frame as a frame of the period of the uplink(link from the terminal to the base station) are continuous and havesymmetric configurations. Here, the term “symmetric” indicates that thedownlink and the uplink have the same period and the same number ofslots.

For example, the frame configuration of FIG. 2 is a configuration of thecase of four time slots (S1 to S4) as in a conventional PHS system usedwidely. The vertical axis denotes the frequency axis and the horizontalaxis denotes the time axis. According to this configuration, it can beincorporated and used in the conventional PHS system.

In FIG. 2, both in the downlink period and the uplink period, it isdivided into 28 frequency bands with respect to the frequency axis. Asubchannel capable of being allocated to the first frequency band iscalled a control subchannel and used in a control channel (CCH).

The above-described first frequency band can be the highest frequencyband or the lowest frequency band.

The example of FIG. 2 is an example of the PHS system. Four basestations can be allocated to control subchannels C₁ to C₄.

Each of the remaining 27 frequency bands (groups) is divided into fourparts for each time slot in the time-axis direction and all 108 subchannels are provided. These are traffic subchannels T₁ to T₁₀₈ fortransmitting and receiving data. That is, in the OFDMA scheme in thecommunication of this embodiment, the number of subchannels (the numberof extra subchannels) is as many as 108 since the subchannels of theconventional OFDMA scheme are divided in the time-axis direction.

The traffic subchannels include subchannels called an anchor subchanneland an extra subchannel.

The anchor subchannel is a subchannel used to provide each terminal witha notification indicating which user uses which subchannel or used forthe base station and terminal to negotiate whether data has beenaccurately exchanged in re-transmission control, and one anchorsubchannel can be allocated to each terminal at a start ofcommunication.

The extra subchannel is a subchannel for transmitting data to be usedsubstantially, and an arbitrary number of extra subchannels can beallocated to one terminal. In this case, as the number of allocatedextra subchannels increases, high-speed communication is possible sincea band extends.

Next, the above-described traffic subchannel allocation will bedescribed. FIG. 3 is an illustrative diagram of one example ofsubchannel allocation. In the example shown in FIG. 3, each trafficsubchannel allocation is shown in various patterns.

In the example shown in FIG. 3, a control channel of a base station ofC₃ among four base stations is shown in control subchannels. Symbols ofC₃, T₂, and the like correspond to FIG. 2.

T₅ is allocated as an anchor subchannel for a terminal of a user 1. T₂,T₄, T₆, T₇, T₈, T₉, T₁₀, T₁₅, T₁₇, T₂₄, . . . , T₁₀₅ are allocated asextra subchannels for the terminal of the user 1. These subchannels arecommon to the downlink and the uplink. Herein, “common” means that atime slot and a frequency band are same in the downlink and the uplink.That is, T₅, T₉, T₁₇, . . . used by a user 1, belong to the first timeslot (S1) in both of the downlink and the uplink, T₂, T₆, T₁₀, . . .belong to the second time slot (S2) in both of the downlink and theuplink, T₇, T₁₅, . . . belong to the third time slot (S3) in both of thedownlink and the uplink, and T₄, T₅, T₂₄, . . . belong to the fourthtime slot (S4) in both of the downlink and the uplink. Further, withrespect to a frequency band, T₂ and T₄, T₆, T₇ and T₈, T₉ and T₁₀, T₁₅,T₁₇, T₂₄, . . . T₁₀₅ are common in both of the downlink and the uplink.

T₂₃ is allocated as an anchor subchannel for a terminal of a user 2.T₁₃, T₁₄, T₁₈, T₂₀, . . . are allocated as extra subchannels for theterminal of the user 2. For the user 2 like the user 1, the subchannelallocation is common to the downlink and the uplink.

T₁, T₃, T₁₁, T₁₂, T₁₉, T₂₁, . . . , T₁₀₇ are used between other basestations and other terminals, and T₁₆, T₂₂, . . . , T₁₀₆, T₁₀₈ areunused subchannels.

As described above, in a frame configuration in the communication systemof this embodiment shown in FIG. 3, the downlink frame as a frame of thedownlink period and the uplink frame as a frame of the uplink period arecontinuous and have symmetric configurations.

Next, the format of a subchannel will be described using FIG. 4.

As shown in FIG. 4, one frequency band includes four downlinksubchannels and four uplink subchannels and a total length on the timeaxis is, for example, 5 ms.

Each subchannel includes PR (PRiamble), PS (Pilot Symbol), and otherfields and a length on the time axis is, for example, 625 μs.

PR is a preamble and a signal for providing synchronizing timing byidentifying the start of frame transmission.

PS is a pilot symbol and a known signal waveform for obtaining a phasestandard to accurately identify an absolute phase, or known data.

A subchannel payload is a part for accommodating data of a physicallayer (PHY).

Next, the format of a downlink physical layer (PHY) will be describedusing FIG. 5.

A subchannel payload of an anchor subchannel includes fields of MAP,ACKCH, PHY payload, and the like. The PHY payload accommodated in asubchannel payload of respective extra subchannel is connected thereto.A CRC field is arranged in the end part of the last extra subchannel.

A bit array accommodated in the above-described MAP field is MAPinformation to be transmitted to a terminal (information indicating anavailable or unavailable subchannel for the terminal), and a number isassigned to a traffic subchannel included in one frame and indicated bya bit string corresponding thereto.

For example, when a bit corresponding to an n^(th) traffic subchannel is“1”, it notifies that the n^(th) traffic subchannel can be allocated andused for a corresponding terminal. When the bit corresponding to then^(th) traffic subchannel is “0”, it notifies that the n^(th) trafficsubchannel cannot be used for the corresponding terminal.

For example, the MAP information in the example of the frameconfiguration of FIG. 3 is as follows.

A bit array of the MAP information to be transmitted to the terminal ofthe user 1 becomes “01010111110000101 . . . 1000”.

A bit array of the MAP information to be transmitted to the terminal ofthe user 2 becomes “00000000000011000101 . . . ”.

Next, the format of an uplink physical layer (PHY) will be describedusing FIG. 6.

A subchannel payload of an anchor subchannel includes fields of RMAP,ACKCH, PC, PHY payload, and the like. The PHY payload accommodated in asubchannel payload of respective extra subchannel is connected thereto.A CRC field is arranged in the end part of the last extra subchannel.

The RMAP is used to send a reply by determining whether or not asubchannel indicated from the base station is available. For example,when another terminal or another base station, and the like existsaround a terminal, a disturbance level by an interference wave therefromis large, and normal communication by a subchannel corresponding theretocannot be performed, a reply indicating that the subchannel isunavailable is sent to the base station. That is, an RMAP bitcorresponding to an unavailable subchannel is set to “0”.

For example, in the case of determining that a terminal side cannot usea third subchannel when MAP information (MAP bit array) transmitted fromthe base station to a certain terminal in the downlink is “10110 . . .”, a third bit is set to “0”.

Accordingly, in this case, a reply of the RMAP of an array of “10010 . .. ” is sent to the base station side in the uplink.

Next, a frame corresponding to transmitted MAP information will bedescribed using FIG. 7.

The base station notifies communication right to the terminal in a MAPfield included in an anchor subchannel in the downlink period of (1) (asindicated by the timing of (1)).

Next, an extra subchannel to be used is indicated in the notified MAPinformation and communication is performed in (the timing of) a frame ofeither (2) or (3) using the extra sub channel whose use is indicated.

Next, whether to perform communication in the frame of either (2) or (3)is determined in an initial step of a connection of the base station andthe terminal. It is determined that a communicable frame is (2) or (3)according to a condition of a terminal in which a demodulation processrate is slow, and the like. Once the determination is made, whether toperform communication in the frame of either (2) or (3) is not changeduntil the communication is terminated.

Since the number of subchannels (the number of extra subchannels) is asmany as 108 in the OFDMA scheme in the communication system of thisembodiment, the number of subchannels capable of being allocated to eachuser is also large. Accordingly, when the MAP information also increasesessentially and the MAP information exchange is also performed betweenthe base station and the terminal in the uplink in addition to thedownlink, a large amount of communication resources is used and apayload for communicating the original data is reduced.

However, since a downlink frame as a frame of a downlink period and anuplink frame as a frame of an uplink period are continuous and havesymmetric configurations in the communication system of this embodimentas shown in FIG. 3, when the subchannel allocation of the uplink iscommon to the downlink, that is, a same slot and a same frequency band,MAP information is notified from the base station to the terminal, forexample, only by the downlink, so that a notification of the MAPinformation in the uplink is unnecessary and throughput can be improvedsince a payload part for communicating the original data can be securedplentifully.

As described above, a communication system according to the embodimentof the present invention includes a downlink frame generation unit 14that generates a downlink frame for a downlink period for performingcommunication from a base station 10 to at least one terminal 20 of aplurality of terminals, and an uplink frame generation unit 24 thatgenerates an uplink frame for an uplink period for performingcommunication from at least one terminal 20 of the plurality ofterminals to the base station 10, wherein the downlink frame and theuplink frame have symmetric configurations.

Thereby, the communication resource reduction can be suppressed and theprocessing load on the base station 10 can be reduced. The compatibilitywith the conventional PHS system can be maintained.

Priority is claimed on Japanese Patent Application No. 2006-257969,filed Sep. 22, 2006, and Japanese Patent Application No. 2006-320776,filed Nov. 28, 2006, the content of which is incorporated herein byreference.

1. An OFDMA communication system for performing communication using oneor more subchannels between a base station and a plurality of terminals,the communication system comprising: a downlink frame generation unitwhich generates a downlink frame for a downlink period for performingcommunication from the base station to at least one terminal of theplurality of terminals; an uplink frame generation unit which generatesan uplink frame for an uplink period for performing communication fromat least one terminal of the plurality of terminals to the base station;a channel allocation unit which allocates, for one terminal of theplurality of terminals, one or more subchannels available in the oneterminal, wherein the channel allocation unit allocates, among thesubchannels, subchannels having a same time slot and a same frequencyband in the downlink frame and in the uplink frame for a single terminalof the plurality of terminals.
 2. The communication system of claim 1,wherein a number of subchannels configuring the downlink frame is sameas a number of subchannels configuring the uplink frame.
 3. Thecommunication system of claim 1, wherein the downlink frame and theuplink frame are arranged continuously.
 4. The communication system ofclaim 1, wherein each of the downlink frame and the uplink frameincludes a control subchannel to be used as a control channel of thebase station and a traffic subchannel for transmitting data, and whereinthe traffic subchannel includes: a first subchannel which is allocatedfor each terminal and includes information indicating an available orunavailable subchannel for each terminal; and a second sub channelincluding data to be used substantially.
 5. The communication system ofclaim 4, wherein the information indicating an availably or unavailablesubchannel for each of the plurality of terminals is included in thefirst subchannel and notified from the base station to the correspondingterminal in the downlink period.
 6. The communication system of claim 4,wherein, among the available subchannels in the information, asubchannel to be used and a subchannel to be unused in each terminal areincluded in the first subchannel while being differentiated from eachother, and notified from the corresponding terminal to the base stationin the uplink period.
 7. An OFDMA communication method for performingcommunication using one or more subchannels between a base station and aplurality of terminals, the communication method comprising; generatinga downlink frame for a downlink period for performing communication fromthe base station to at least one terminal of the plurality of terminals,and an uplink frame for an uplink period for performing communicationfrom at least one terminal of the plurality of terminals to the basestation; and allocating, for one terminal of the plurality of terminals,one or more subchannels available in the one terminal, whereincommunication is performed by allocating, among the subchannels,subchannels having a same time slot and a same frequency hand in thedownlink frame and in the uplink frame for a single terminal of theplurality of terminals.
 8. The communication method of claim 7, whereina number of subchannels configuring the downlink frame is same as anumber of subchannels configuring the uplink frame.
 9. The communicationmethod of claim 7, further comprising notifying information indicting anavailable or unavailable subchannel for each of the plurality ofterminals to the corresponding terminal in the downlink period.
 10. Thecommunication method of claim 9, further comprising, after notifying theinformation to each terminal in the downlink period, notifying, amongthe available subchannels, a subchannel to be used and a subchannel tobe unused in each terminal while being differentiated from each other,from the corresponding terminal to the base station in the uplinkperiod.